1. Field of the Invention
The present invention relates to a measuring apparatus to measure the purity, mixing ratio, and the like of an insulative (nonconductive) fluid flowing through a passage, a purity controller to control the purity of the insulative fluid according to the measurement, and a mixing ratio controller to control the mixing ratio of the insulative fluid according to the measurement.
2. Description of the Related Art
FIG. 10 shows an example of an apparatus to measure a state of a fluid flowing through a pipe according to a prior art. The prior art inserts electrodes 203 and 205 into the pipe 201 and measures a state of a fluid 207 flowing through the pipe 201. More precisely, the electrodes 203 and 205 detect a conductivity in the pipe 201 and finds a conductivity difference between a clean fluid 209 and foreign matter 211 in the fluid 207. If the foreign matter 211 is found in the fluid 207 according to the detected conductivity difference, the fluid 207 with the foreign matter 211 is branched and discharged, the foreign matter 211 is removed to provide the clean fluid 209.
The prior art of FIG. 11 is unable to detect insulative foreign matter contained in an insulative fluid.
Ultrapure water is an insulative (nonconductive) fluid and is employed to wash semiconductor silicon wafers. After the washing, the used ultrapure water is regenerated and used again for wafer washing. The used ultrapure water contains fine silicon-wafer fragments. If the ultrapure water containing the silicon fragments is used as it is to wash wafers, the surfaces of the wafers will be damaged to deteriorate the yields of semiconductor products. The fragments, therefore, must be removed. To remove the fragments from the used ultrapure water, filters are employed. Before using the regenerated ultrapure water for washing wafers, it is inspected for foreign matter. If foreign matter is detected in the regenerated ultrapure water, the water is again passed through the fillers, and only ultrapure water containing no foreign matter is used for washing wafers.
Ultrapure water and wafer fragments are insulative or nonconductive, and therefore, the prior art of FIG. 11 that measures a conductivity is incapable of detecting silicon fragments in ultrapure water.
To mirror-finish semiconductor silicon wafers, abrasive (slurry) is employed. The slurry is a mixture of ultrapure water and silica particles. The silica is a kind of aluminum ceramics. If a silica concentration in the ultrapure water is low, the slurry will insufficiently mirror-finish the wafers, and if the silica concentration is too high, the slurry will damage mask patterns on the wafers. The silica concentration of the slurry, therefore, must correctly be controlled. The ultrapure water and silica particles are insulative or nonconductive, and therefore, the prior art of FIG. 11 that measures a conductivity is incapable of detecting a concentration of silica particles in ultrapure water.
The slurry is regenerated and repeatedly used, and like the ultrapure water used for washing wafers, the slurry is inspected for foreign matter such as silicon fragments before reuse, so that only slurry containing no foreign matter is used to polish wafers. The prior art of FIG. 11 is unable to detect such foreign matter in slurry due to the same reason mentioned above.
There are other measuring apparatuses employing ultrasonic waves, visible rays, UV monitors, and the like for measuring insulative fluids. These apparatuses suffer from low accuracy, and therefore, are insufficient to increase the yields of wafer processing.